1. Field of the Invention
The present invention relates to a temperature monitoring unit useful for monitoring the heated state of a rotary heating element of an air preheater in a thermal power plant, etc.
2. Description of the Related Art
A mirror is provided within a duct of an air inlet of a conventional temperature monitoring unit for monitoring the heated state of a rotary heating element of a rotary regenerative air preheater. The mirror reflects light of a thermal point (abnormally high temperature portion) of the heating element in the radial direction of the heating element and is movable in the radial direction to scan the heating element. Devices converge the reflected light toward an infrared sensor via a circular lens (about 150 .phi.). The light is image-processed and the output of the infrared sensor is so displayed.
However, the prior art unit presents the following problems.
a) The sensitivity of the device for converging and detecting the reflected light from the heating element via the circular lens is restricted by the diameter of the lens when a thermal point (abnormally high temperature portion) of a heating element of a large power generation unit is to be detected. Therefore, the scanning of the entire area of the heating element having a large diameter takes a long time, whereby a fire may not be detected in its early stage.
b) In order to shorten the scan time, the use of a plurality of circular lenses (diameter 150 .phi.) or of a large diameter lens may be considered. However, the former idea requires a space for accommodating or storing the mirrors within the duct to be large, which is not desirable. The latter idea is not preferable in terms of the function of the unit because the visual field of detection is enlarged and the sensitivity of the infrared sensor for detecting the thermal point, i.e. the ratio S/N of an area of the thermal point to an area of the lens, is lowered.
c) The prior art unit cannot determine whether the surface of the sensor is contaminated or the sensor is defective when the output of the intensity of infrared rays becomes even abnormally low.
d) Generally a highly sensitive infrared sensor is very sensitive to environmental temperature and its output drifts regardless of the intensity of the incident light due to the temperature of the sensor itself or to infrared rays radiating from the inner wall of the sensor housing. Accordingly, a correct temperature of the object cannot be detected. This is a problem especially when the amount of incident light is very small because its signal and drift become relatively very close.